Real and apparent depth

The refraction of light at the surface of water
makes ponds and swimming pools appear shallower than they really are. A 1m deep pond
would only appear to be 0.75 m deep when viewed from directly above.

When light emerges from glass or water into air it speeds up
again. If it meets the glass-air boundary at any angle other than 0o it will refract away from
the normal. This is true for small angles – something else happens when the angles get
larger.

If you look at a stick that is poking into some water at an angle the stick looks
bent because of refraction. The bottom of the stick seems to be nearer the surface of the
water than it really is. It also explains why flat-based swimming pools appear to get shallower
as you look towards the end furthest from you.

There is a connection between the
real and apparent depths of the water. It can be proved that: